1. Field of the Invention
This invention relates to a method of measuring the incremental environmental exposure of an indicator device that includes a tuned circuit and an element that is sensitive to the environmental exposure.
2. Background of the Invention
Several patents have disclosed methods for measuring environmental exposure.
U.S. Pat. No. 4,189,399, issued Feb. 19, 1980, to Patel, discloses co-crystallized acetylenic compounds useful in measuring time-temperature or radiation-dosage history of an article by a color change.
U.S. Pat. No. 4,212,153, issued July 15, 1980, to Kydonieus et al., discloses a two-layer time-temperature indicator that changes color as the interior layer migrates to the outer surface of the exterior layer.
European Patent Application, Publication No. 0117390, published Sept. 5, 1984, discloses a process for measuring environmental exposure of indicator devices that comprise a composition whose optical reflectivity changes incrementally with incremental environmental exposure.
A characteristic of these prior art methods is that the indicator reacts to the environmental exposure with a change in color and/or optical reflectivity. A quantitative measure of the exposure requires an optical detector that can "see" the indicator. Thus, if the indicator is monitoring the exposure of a product, it must be at the surface of the product in order to be read. If the product is packaged, the prior art methods measure the environmental exposure of the outer surface of the packaging, which may be substantially different from the exposure of the product.